These years, a growing number of trucks and buses use heavy-duty tires that employ a single tire installation with a low-profile structure for the purpose of reducing fuel consumption and saving natural resources. The most widely employed practice for tires each with an aspect ratio of 60% or less is to dispose a circumferential-direction reinforcement layer, whose cord angle to the circumferential direction of the tire is substantially 0 degrees, between cross belt layers (see Patent Document 1, for example). In such a belt structure, a tension of each cord in each edge portion of the circumferential-direction reinforcement layer is high. For this reason, the cords in each edge portion of the circumferential-direction reinforcement layer tend to break due to their fatigue. To avoid this, the cross belt layers are formed larger in width than the circumferential-direction reinforcement layer, and laminated to be in direct contact with each other at outer sides, in the width direction, of the circumferential-direction reinforcement layer. The cross belt layers work at the outer sides, in the width direction, of the circumferential-direction reinforcement layer so as to reduce the tension of each cord in each edge portion of the circumferential-direction reinforcement layer. This tension reduction consequently suppresses the fatigue break of the cords.
However, in the case where the cross belt layers are formed larger in width than the circumferential-direction reinforcement layer, and where such cross belt layers are laminated to be in direct contact with each other at outer sides, in the width direction, of the circumferential-direction reinforcement layer, the total width of the circumferential-direction reinforcement layer is restricted. As a result, the circumferential-direction reinforcement layer does not always bring a sufficient effect of improving the high-speed durability of the tire. In addition, because the cross belt layers are laminated to be in direct contact with each other at the outer sides, in the width direction, of the circumferential-direction reinforcement layer, the shear strain which each cross belt layer receives is large. As a result, the cords and a rubber portion tend to be separated from one another in each edge portion of the cross belt layer.    Patent Document 1: Japanese patent application Kokai publication No. 2001-522748